If it's a head gasket leak can it be changed without pulling the motor?

Probably can be done with the engine in the car but not a good job.

You can confirm it is a head gasket with a BlockTester kit, since it's obviously affecting the coolant. A lot of motors these days are dropped from underneath along with the sub frame, rather than pulled from the top. But to do a big job like this the front of the motor needs to come apart as well as the heads, and to leave motor in your still taking apart practically the entire front clip to get to all that. Talk to a Subaru mechanic or a dedicated Subaru forum and see what's involved. I think most shops would prefer to leave motor in place for just head gaskets. Depends to on how much wear is on engine, piston blow by or worn bearings would necessitate a full rebuild anyhow.

To properly prep the surfaces you really should pull the engine. You also need to send out the heads to be inspected for crack and warp.